1. Field of the Invention
The present invention relates to an image sensing apparatus and a control method therefor, and to an image processing apparatus and a noise reduction method, and more particularly, to an image sensing apparatus for reducing noise caused by driving an image sensor, and a control method therefor, and to an image processing apparatus and a noise reduction method.
2. Description of the Related Art
Recently, image sensing apparatuses such as digital cameras have been actively developed that record and play back images sensed by solid-state image sensors such as CCDs, with the use of, as a recording medium, a memory card including a solid-state memory element, and have become widespread. Behind such wide diffusion lies the spread of personal computers (PCs) that are able to input and process images shot with a digital camera, increased demand for digital image information through the utilization of the Internet or the like, and so on.
Then, digital still cameras, which function mainly to store still images, and digital video cameras, which function mainly to store moving images, each have started to have the ability to store both still images and moving images. Accordingly, market needs for improvement in resolution and operating speed for still image and moving image shooting have been increasing year by year.
Moreover, recently, in addition to improvements in resolution and operating speed, the ability to carry out shooting easily with fewer failures has been required ever more in a variety of shooting scenes. Therefore, shutter speed has been increased in order to follow objects moving fast, such as (for example) athletes in motion or for the purpose of avoiding blurring of images due to hand movement in indoor shooting under low-intensity illumination. Furthermore, in order to enable high-sensitivity shooting in areas in which no flash photography is allowed, such as museums and aquariums, the high degree of sensitivity involved in still image and moving image shooting has become even greater.
On the other hand, in image sensing apparatuses using solid-state image sensors such as CCDs, a driving method for reading is used in which transfer by vertical transfer registers is carried out in parallel with transfer of electric charge of pixels in an effective area from horizontal transfer registers in order to increase the continuous shooting speed in still image shooting and the frame rate in moving image shooting. However, this driving method has the problem of crosstalk noise being superimposed on signals of pixels in an effective area (hereinafter, referred to as “effective pixel signals”) read out from the horizontal transfer registers, at the timings of rising and dropping driving pulses for vertical transfer (vertical transfer pulses).
It is difficult to prevent the above-described crosstalk noise due to the vertical transfer pulses from being superimposed, because the noise leaks into the effective pixel signals through various transmission paths, such as a path between substrates or between signal electrodes inside an image sensor. Furthermore, the above-described crosstalk noise due to the vertical transfer pulses is disadvantageous in being also more likely to be visually noticeable as image noise, because the crosstalk noise will be vertical noise superimposed on effective pixel signals in the same column positions for each horizontal line.
Now, an example of the configuration of the conventional image sensor described above will be described. FIG. 8 is a diagram illustrating a schematic configuration of an image sensor (CCD).
In the figure, reference 1 denotes PD (photo diodes) that are photoelectric conversion elements, which are two-dimensionally arranged in rows and columns. Reference numeral 2 denotes a vertical CCD (VCCD), which refers to multiple vertical transfer registers for transferring the signal charge of the PD 1 in the vertical direction (column direction), and typically has a four-phase drive configuration.
Reference numeral 3 denotes a horizontal CCD (HCCD) for transferring signal charge for each line transferred from the VCCD 2, and typically has a two-phase drive configuration. Reference numeral 4 denotes an output amplifier for converting signal charges for each pixel transferred from the HCCD 3 into a voltage signal, and outputting the voltage signal.
Reference numeral 5 denotes buffer storage cells (BS) for temporarily accumulating signal charge for one line transferred from the VCCD 2 until being transferred to the HCCD 3, and reference numeral 6 denotes transfer gates (TG) between the BS 5 and the HCCD 3.
In FIG. 8, vertical transfer pulses V1, V2, V3, V4 are respectively applied to four transfer electrodes of the VCCD 2, whereas horizontal transfer pulses H1, H2 are respectively transferred to two transfer electrodes of the HCCD 3.
FIG. 9 is a timing chart for explaining a conventional image sensing drive method for the image sensor having the configuration shown in FIG. 8, which shows timings of outputting signal charges of the CCDs to the output amplifier 4 by the VCCD 2 and the HCCD 3.
Further, as shown in the timing chart of FIG. 9, the horizontal blanking period (HBLK) is reduced in such a way that the operation of signal charge transfer by the VCCD 2 is carried out temporally in parallel with the horizontal effective operation period, that is, the transfer operation by the HCCD 3.
However, every time the transfer operation by the VCCD 2 is carried out during the horizontal effective operation period, crosstalk noise will be generated inside the CCDs at the timings of rising and dropping each of the vertical transfer pulses V1, V2, V3, V4 applied to the VCCD 2, thereby resulting in the crosstalk noise superimposed on image sensing voltage signals (CCD outputs) eventually read out from the output amplifier 4.
The crosstalk noise due to the vertical transfer pulses are superimposed as about the same level of noises, always on the pixels in the same horizontal positions for each horizontal line, as indicated by downwards arrows in FIG. 9. Therefore, the crosstalk noise appears as vertical noise on two-dimensional images of the horizontal lines arranged. Furthermore, the read image sensing voltage signals are amplified by a gain amplifier, not shown in the figure, for switching the sensitivity, which is located at the subsequent stage to the output amplifier 4. Therefore, in a case in which the sensitivity is set higher in the image sensing apparatus, that is, in a case in which the amplification of the gain amplifier is larger, the crosstalk noise will become recognizable and more likely to be noticeable.
In order to deal with the problems described above, a technique is disclosed for achieving reduction in read time and improvement in frame rate by using the driving method described above during the period for reading out unneeded electric charges only when partial reading is carried out from an image sensor (see Japanese Patent No. 3715781)
Furthermore, a technique is disclosed for reducing the influence of crosstalk noise superimposed on effective pixel signals by reducing the slopes of, or controlling the timing of, rising and dropping of the vertical transfer pulses (see Japanese Patent Laid-Open No. 2005-269060).
However, while the technique disclosed in Japanese Patent No. 3715781 is effective in a case in which reading from a portion of an effective pixel area is carried out, such as in photometric operation and electronic zoom operation, speeding up is not possible in the operation of reading out normal still images or moving images.
Furthermore, the technique described in Japanese Patent Laid-Open No. 2005-269060 fails to completely eliminate the influence of the crosstalk noise superimposed on the effective pixel signals, thereby producing residual noise components. In a case in which the sensitivity condition is set higher in the image sensing apparatus and the amplification is larger for image sensing signals of the image sensing circuit, even faint noise components will be recognizable as image noise. This becomes a factor hindering increase in the sensitivity of the image sensing apparatus.